Lamps and displays using LEDs (light emitting diodes) for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as fluorescent lamps, including low power consumption, long lifetime, and no hazardous material, and additional specific advantages for different applications. For instance, LEDs are rapidly replacing Cold Cathode Fluorescent Lamps (CCFL) as LCD backlights due to smaller form factor and wider color gamut. LEDs for general illumination provide the opportunity to adjust the color or white color temperature for different effects. LED billboards are replacing paper billboards to allow multiple advertisements to timeshare a single billboard. Further, projectors that use LEDs as the light source may become popular in mobile handsets, such as smartphones, in the near future. Likewise, organic LEDs or OLEDs, which use multi-colored LEDs directly to produce light for each display pixel, and which use arrays of organic LEDs constructed on planar substrates, may also become popular for many types of display applications.
With such diverse and large markets for LEDs as light sources, many semiconductor companies offer a wide range of LED driver products. Manufacturers include Analog Devices, Freescale, Infineon, Maxim Integrated Products, Linear Technologies, On Semiconductor, Supertex, ST Microelectronics, NXP, National Semiconductor, Texas Instruments, Toshiba, and Sharp among others. LED driver products range from arrays of current sources to drive many LEDs independently, as is required for a digital billboard, to switch mode power supplies to produce the high voltage used to drive a string of many LEDs serially connected, as is used in the backlight of a small computer with an LCD display, to sophisticated systems that include arrays of currents sources and power supplies to drive many LEDs and sensors and that include controllers to monitor temperature and light and adjust LED drive currents accordingly.
Although it is not widely known, LEDs not only produce light when current is applied, but LEDs also produce a current when exposed to light. As such, LEDs can be used to transmit light, and LEDs can be used to receive light. To transmit light, control circuitry can be used to produce high currents to drive the LEDs so that they emit light. To receive light, control circuitry can be used to detect the very small currents produced by the LEDs when light is incident on the LEDs.
Common circuitry to detect very small currents in optical detectors, such as silicon photo -diodes, often includes a trans-impedance amplifier, which when connected to the detector properly maintains a fixed voltage across the detector and generates a voltage proportional to the current induced in the detector by incident light. Such trans-impedance amplifiers specifically designed for optical detectors are available from semiconductor suppliers such as NXP, Texas Instruments, Maxim Integrated Products, and Analog Devices. However, difficulties can exist in implementing high performance optical receivers using such trans-impedance amplifiers. For example, because the currents produced by optical detectors are very small, the circuitry can be sensitive to noise coupled into the detection circuitry from the power supply.
U.S. Pat. No. 7,072,587 owned by Mitsubishi describes using an LED to transmit and receive light. In particular, the circuitry described uses a general purpose input/output (I/O) pin of a microcontroller connected to a serial combination of an LED and a resistor. As such, when emitting light, the power supply voltage to the microcontroller is applied to the LED and resistor, which results in a current through the LED that varies at least with power supply voltage and temperature, and consequently the light produced by the LED varies with the current. As described in this Mitsubishi patent, when the LED is detecting light, current induced in the LED forward biases the LED to produce a voltage that is either above or below a threshold that the microcontroller detects. The time taken to charge the LED to this threshold is measured to determine the intensity of incident light. Although relatively simple, the light detection circuitry described in this Mitsubishi patent is relatively slow and is susceptible to many sources of error. For instance, as the power supply voltage to the microcontroller varies, so does the threshold voltage. As such, little or no power supply rejection exists. Additionally, the capacitance of the LED, may vary from part to part and over temperature, which adversely affects the photo-current measurements. Further, leakage currents through the transistors in the microcontroller I/O pins, which vary exponentially with temperature, combine with the light induced current to change the LED capacitance and also adversely affect photo-current measurements.
As such, a need exists for electronic implementations that can provide high currents to emit light from an LED or LEDs and that can detect small currents to measure light incident on the same LED or LEDs while also reducing the effects of power supply noise and leakage currents.